AC coupling modules for bias ladders

ABSTRACT

A positive-logic FET switch stack that does not require a negative bias voltage, exhibits high isolation and low insertion/mismatch loss, and may withstand high RF voltages. Embodiments include a FET stack comprising series-coupled positive-logic FETs (i.e., FETs not requiring a negative voltage supply to turn OFF), series-coupled on at least one end by an “end-cap” FET of a type that turns OFF when its V GS  is zero volts. The one or more end-cap FETs provide a selectable capacitive DC blocking function or a resistive signal path. Embodiments include a stack of FETs of only the zero V GS  type, or a mix of positive-logic and zero V GS  type FETs with end-cap FETs of the zero V GS  type. Some embodiments withstand high RF voltages by including combinations of series or parallel coupled resistor ladders for the FET gate resistors, drain-source resistors, body charge control resistors, and one or more AC coupling modules.

CROSS-REFERENCE TO RELATED APPLICATIONS—CLAIM OF PRIORITY

The present application is a continuation of, and claims priority to, commonly assigned U.S. patent application Ser. No. 16/852,804, filed Apr. 20, 2020, entitled “AC Coupling Modules for Bias Ladders”, to issue on May 25, 2021 as U.S. Pat. No. 11,018,662, the contents of which is incorporated herein by reference. Application Ser. No. 16/852,804 is a continuation of, and claims priority to, commonly assigned U.S. patent application Ser. No. 16/261,167, filed Jan. 29, 2019, entitled “AC Coupling Modules for Bias Ladders”, now U.S. Pat. No. 10,630,280 issued Apr. 21, 2020, the contents of which is incorporated herein by reference. Application Ser. No. 16/261,167 is a continuation of, and claims priority to, commonly assigned U.S. patent application Ser. No. 15/939,144, filed Mar. 28, 2018, entitled “AC Coupling Modulesfor Bias Ladders”, now U.S. Pat. No. 10,236,872, issued Mar. 19, 2019, the contents of which is incorporated herein by reference.

The present application may be related to the following patents and patent applications, the contents of all of which are incorporated herein by reference:

-   -   U.S. Pat. No. 8,669,804, issued on Mar. 11, 2014 and entitled         “Device and Methods for Improving Voltage Handling and/or         Bi-directionality of Stacks of Elements when Connected Between         Terminals”;     -   U.S. patent application Ser. No. 15/256,453, filed Sep. 2, 2016         and entitled “Positive Logic Digitally Tunable Capacitor”, now         U.S. Pat. No. 9,948,281 issued Apr. 17, 2018;     -   U.S. patent application Ser. No. 15/289,768, filed Oct. 10,         2016, entitled “Reduced Dissipation Switch FET Gate Biasing”,         now abandoned, which claims priority to U.S. Provisional         Application No. 62/241,498, filed Oct. 14, 2015;     -   U.S. patent application Ser. No. 15/939,128, filed on Mar. 28,         2018, entitled “Positive Logic Switch with Selectable DC         Blocking Circuit”;     -   U.S. patent application Ser. No. 15/939,132, filed on Mar. 28,         2018, entitled “Stacked FET Switch Bias Ladders”.

BACKGROUND (1) Technical Field

The invention relates to electronic circuits, and more particularly to radio frequency electronic switch circuits and related methods.

(2) Background

In radio frequency (RF) systems, such as cellular phones and WiFi networks, electronic switch circuits are often used in series with RF signal lines to selectively block or conduct RF signals, such as between an antenna and a transceiver circuit. Electronic switch circuits are also often used in a shunt configuration between an RF signal line and a reference potential (e.g., circuit ground), for example, to selectively isolate nodes of the RF signal line from significantly influencing other circuitry. As one example, FIG. 1 is a simplified schematic circuit of a common prior art series-shunt switch circuit configuration 100. In the illustrated configuration, a series switch circuit 102 is coupled in series with an RF signal line 104 between an RF_(IN) port and an RF_(OUT) port, and a shunt switch circuit 106 is coupled to the RF signal line 104 between the RF_(OUT) port and circuit ground. As illustrated, the series switch circuit 102 and shunt switch circuit 104 are modeled as single-pole, single-throw (SPST) switches. In use, the series switch circuit 102 and shunt switch circuit 106 are generally operated in a complementary manner: if series switch circuit 102 is open, then shunt switch circuit 106 is closed, and if series switch circuit 102 is closed, then shunt switch circuit 106 is open.

Ideally, switch circuits such as those shown in FIG. 1 should not appreciably alter or affect an RF signal. However, in integrated circuits, RF switching circuits are generally implemented with transistors, particularly field-effect transistors (FETs), and more particularly MOSFETs. A FET in a conducting (ON) state presents some resistance, R_(ON), to a conducted signal, and in a blocking (OFF) state presents some capacitance, C_(OFF), which may be in series, shunt, or parallel to an RF signal line. Accordingly, FET-based switch circuits generally behave less than ideally in an RF circuit.

A further issue with FETs is that the voltage that a single FET can withstand between drain and source without breaking down is generally limited to a few volts. In an electronic system, there may be parts of the system where the voltage that must be withstood far exceeds the voltage handling capability of a single FET. A common solution is to series stack FETs so that the drain-source voltage across any one FET is less than its drain-source breakdown voltage.

For example, FIG. 2 is a schematic circuit of a prior art series-shunt RF switch circuit 200 using FET stacks. In this example, an RF signal from an antenna ANT is coupled from RF_(IN) through a series stack 202 to RF_(OUT). A shunt stack 204 is coupled between RF_(OUT) and circuit ground. Each of the stacks 202, 204 comprises a plurality of series-coupled FETs M1-Mn (although “n” may differ for the two stacks 202, 204). In this example, each FET includes a dedicated gate resistor Rg.

A standard biasing scheme is usually used in this type of circuit 200 when configured with FET switches having a common threshold voltage and channel structure. A positive voltage is applied to the gate of each FET M1-Mn of a stack 202, 204 to turn the stack ON (conducting), and a negative voltage is applied to the gate of each FET M1-Mn of a stack 202, 204 to turn the stack OFF (blocking). By way of example, a positive DC voltage of +3 volts may be applied (through the resistors Rg) to the gates of each FET M1-Mn of a stack 202, 204 to turn the stack ON, and a negative DC voltage of −3 volts may be applied (again through the resistors Rg) to the gates of each M1-Mn of a stack 202, 204 to turn the stack OFF. For good performance, it is desirable to use a large enough negative voltage to fully turn each FET switch M1-Mn OFF. Turning the FET switches into a full OFF state results in improved RF linearity in terms of harmonics and inter-modulation distortion (IMD), better RF power handling, higher isolation, and lower leakage current, among other advantages.

In most applications using an integrated circuit (IC) implementation of the circuit shown in FIG. 2, a positive supply voltage is readily available from an external source. A negative supply voltage is usually generated on-chip using a bias generator 206, shown in FIG. 2 as coupled to the gate resistors Rg. The bias generator 206 may conventionally use one or more negative voltage charge pumps and voltage regulators that convert an externally supplied positive supply voltage to a negative voltage.

The configuration shown in FIG. 2 works reasonably well for many applications. However, there are applications where generating a negative supply voltage is either not desired or not practical due to design constraints; in some applications, use of a negative charge pump is not even possible. These are generally applications with stringent requirements such as extremely low current and power consumption, extremely low noise sensitivity, and/or very small IC die areas. Moreover, in biasing schemes wherein charge pumps are used to generate negative supply power, switching speed is limited by the current sourcing capability of the charge pumps. Further, the voltage swing on the FET gates from full ON to full OFF is significant—from +3V to −3V in the example above, or a 6V swing, which further limits switching speed. In addition, the charge swing has to go through the series gate resistors of the FETs, which dampens and slows the switching process.

Solutions to utilizing a FET switch stack without using a negative supply voltage are taught in co-pending U.S. patent application Ser. No. 15/256,453, filed Sep. 2, 2016 and entitled “Positive Logic Digitally Tunable Capacitor”. FIG. 3 is a schematic circuit of a series-shunt RF switch circuit 300 using “positive logic” FET stacks. In accordance with one embodiment of the referenced patent application, an RF signal from an antenna ANT is coupled from RF_(IN) through a series stack 302 to RF_(OUT). A shunt stack 304 is coupled between RF_(OUT) and circuit ground. Each of the stacks 302, 304 comprise a plurality of series-coupled FETs M1-Mn (although “n” may differ for the two stacks 302, 304) which have the characteristic that each FET turns OFF sufficiently when Vgs≈0V. In this example, each FET includes a dedicated gate resistor Rg. Each stack 302, 304 also includes two bracketing DC blocking capacitors C connected in series with the FETs M1-Mn; the blocking capacitors C may each have different values for particular applications.

Each of the stacks 302, 304 is configured to be coupled to two non-negative supply voltages, Vgate and Vdrain, which are used to control the switch state, ON or OFF, of associated FETs M1-Mn. The Vgate supply voltage is coupled to the gates of the FETs M1-Mn of each stack 302, 304 through corresponding parallel gate resistors Rg. The Vdrain supply voltage is coupled to a network of parallel resistors Rds that are coupled as shown to respective drains or sources of each of the FETs M1-Mn, including shared drains-source nodes between the FETs M1-Mn.

In the illustrated example, the FETs M1-Mn are turned ON by applying a positive Vgate voltage (e.g., +3V) to the FET gates through their respective gate resistors Rg, while applying a Vdrain voltage of 0V to the FET drains and sources through respective Rds resistors. This configuration effectively creates a positive DC gate-source voltage, V_(GS), for every FET M1-Mn (i.e., V_(GS)=+3V with respect to the source voltage of 0V if Vgate=+3V). The FET stacks are turned OFF by applying a Vgate voltage of 0V to the FET gates through their respective gate resistors Rg, while applying a positive Vdrain voltage (e.g., +3V) to the FET drains and sources through respective Rds resistors. This effectively creates, in relative terms, a negative V_(GS) for every FET M1-Mn (i.e., V_(GS)=−3V with respect to the source voltage of +3V if Vgate=0V).

Obtaining a negative V_(GS) for the OFF state is accomplished by virtue of the DC blocking capacitors C on both ends of each stack 302, 304 to achieve the required gate-source voltage levels in both the ON and OFF states. Without the blocking capacitors C, the “end” FETs (i.e., M1, Mn) would have either a drain or source DC coupled to some potential (e.g., circuit ground) that would not allow the required V_(GS) voltage to be applied to the FETs M1-Mn. With the DC blocking capacitors C, when a positive Vdrain voltage is applied, a positive voltage can be built up on the drain and source nodes of the FETs M1-Mn as the capacitors C charge. Other variations and modes of operation are also disclosed in U.S. patent application Ser. No. 15/256,453.

Circuits of the type shown in FIG. 3 avoid use of a negative voltage and thus require no negative voltage charge pump. Switching speed is increased because discharge of the FET gates does not have to go through a charge pump. Accordingly, operation without a charge-pump enables a zero current standby mode, with very fast switching and settling times and a valid RF state in standby mode, as well as other advantages as disclosed in the U.S. patent application Ser. No. 15/256,453. However, there is room for improvement.

In particular, the DC blocking capacitors C have to be large for high isolation and for low insertion loss and low mismatch loss (i.e., their impedance at a desired operating frequency needs to be low), thus consuming IC die area and increasing cost. The constant presence of the DC blocking capacitors C in both ON and OFF states for a series stack 302 affects a conveyed RF signal, while their constant presence and size impacts isolation. Further, the constantly present DC blocking capacitors C have to be charged through the Rds resistor network, which impacts switching and settling times (this is in addition to the usual Rg*Cgs time constant on the gate terminal side). Moreover, the DC blocking capacitors C are generally formed as metal-insulator-metal (MIM) capacitor structures, which are sensitive to electro-static discharge (ESD) and thus require ESD protection on RF ports (e.g., RF_(IN)).

In addition, in practical applications, a series stack 302 generally requires an integrated shunt tuning inductor L coupled near each DC blocking capacitor C to perform both impedance matching (i.e., resonating out the capacitance of the DC blocking capacitors C) and ESD protection for port connections (by providing a DC path to a reference potential). Such added shunt tuning inductors L, in combination with the adjacent blocking capacitors C, cause the series stack 302 to have a high-pass filter response, affecting the RF performance of the switch. The shunt tuning inductors L also consume IC area, thus increasing cost.

A further area for improvement is in handling high voltage. Embodiments of the type shown in FIG. 3 may breakdown when subjected to a high voltage RF signal due to RF voltages being coupled through to gate resistors, causing breakdown.

Accordingly, there is a need for an improved FET switch stack that does not require a negative bias voltage, but which also avoids or mitigates the drawbacks of positive logic FET stacks as described above, and which can withstand, in some embodiments, application of a high voltage RF signal. The present invention addresses these and other needs.

SUMMARY

The invention encompasses an improved positive-logic FET switch stack that does not require a negative bias voltage, exhibits high isolation and low insertion loss and mismatch loss characteristics, requires less integrated circuit (IC) area, and in some embodiments, can withstand application of a high voltage RF signal.

Embodiments include a FET stack comprising one or more series-coupled positive-logic FETs M1-Mn (i.e., zero-Vt, low-Vt, or high-Vt FETs of a type that require a negative V_(GS) to turn OFF but configured so as to not require a negative power supply), series-coupled on at least one end to an “end-cap” FET M₀, where “M₀” designates a FET of a high-Vt type that turns OFF when the V_(GS) of such FET is essentially zero volts. The end-cap M₀ FETs selectably provide either a capacitive DC blocking function or a resistive signal path. Some embodiments may comprise a stack of only M₀ FETs, or a mix of positive-logic FETs and M₀ FETs, so long as at least one end-cap FET is an M₀ FET. Optional end-cap capacitors may be included to prevent early breakdown of corresponding end-cap M₀ FETs.

Some embodiments are configured to withstand high applied RF voltages by including a series-connected bias resistor ladder for the gates of the FETs in the stack (i.e., M1-Mn, M₀), a series-connected Rds drain-source resistor ladder, and either a series-connected or a parallel-connected Rb body charge control resistor ladder. Some embodiments may use series-connected Rb body charge control resistor ladder in conjunction with a parallel-connected bias resistor ladder for the gates. Some embodiments may use a series-connected bias resistor ladder (in a “rail” configuration) in conjunction with a parallel-connected bias resistor ladder (in a “rung” configuration) for the gates.

Some embodiments are configured with one or more AC coupling modules each coupled to at least one end of a corresponding gate bias resistor ladder and/or body charge control resistor ladder, and configured to be coupled to a radio frequency voltage source.

The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic circuit of a common prior art series-shunt switch circuit configuration.

FIG. 2 is a schematic circuit of a prior art series-shunt RF switch circuit using FET stacks.

FIG. 3 is a schematic circuit of a series-shunt RF switch circuit using “positive logic” FET stacks.

FIG. 4 is a schematic circuit of an improved positive logic FET stack.

FIG. 5A is a table showing example voltages for Vgate and Vdrain corresponding to ON and OFF states for the FET stack of FIG. 4.

FIG. 5B is a simplified schematic circuit of the FET stack of FIG. 4 showing example drain and source voltages for the interior FETs Mn and end-cap FETs M₀ when in an ON state.

FIG. 5C is a simplified schematic circuit of the FET stack of FIG. 4 showing example drain and source voltages for the interior FETs Mn and end-cap FETs M₀ when in an OFF state.

FIG. 6 is a schematic diagram of a series-shunt RF switch circuit using the improved positive logic FET stacks of FIG. 4.

FIG. 7 depicts a schematic equivalent circuit of a portion of a prior art FET stack, showing parasitic capacitances.

FIG. 8A is a schematic equivalent circuit of a FET stack with a series-connected resistor ladder gate bias scheme in accordance with one aspect of the present invention, showing parasitic capacitances.

FIG. 8B is a schematic diagram of a positive logic FET stack with a series-connected gate bias resistor ladder and a series-connected drain-source resistor ladder in accordance with the present invention.

FIG. 8C is a schematic diagram of a positive logic FET stack with an added series-connected resistor ladder for controlling body charge.

FIG. 8D is a schematic diagram of a conventional FET stack with a series-connected gate bias resistor ladder, a series-connected drain-source resistor ladder, and a series-connected body charge control resistor ladder.

FIG. 8E is a schematic diagram of a conventional FET stack with a series-connected gate bias resistor ladder, a series-connected drain-source resistor ladder, and a parallel-connected body charge control resistor ladder.

FIG. 8F is a schematic diagram of a conventional FET stack with a series-connected gate bias resistor ladder and an AC Coupling Gate Module.

FIG. 8G is a schematic diagram of a conventional FET stack with a series-connected gate bias resistor ladder and dual AC Coupling Gate Modules.

FIG. 8H is a simplified schematic diagram of an SOI MOSFET that includes an ACS structure along with conventional drain D, source S, and gate G terminals and a gate resistor Rg.

FIG. 9 is a process flow diagram showing one method for providing a FET switch stack.

FIG. 10 is a process flow diagram showing one method for providing a series-shunt switch circuit.

FIG. 11 is a process flow diagram showing one method for biasing a FET switch stack.

FIG. 12 is a process flow diagram showing a method for providing a FET switch stack.

FIG. 13 is a process flow diagram showing another method for biasing a FET switch stack.

Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION

The invention encompasses an improved positive-logic FET switch stack that does not require a negative bias voltage, exhibits high isolation and low insertion loss and mismatch loss characteristics, requires less integrated circuit (IC) area, and in some embodiments, can withstand application of a high voltage RF signal.

Embodiments include a FET stack comprising one or more series-coupled positive-logic FETs M1-Mn (i.e., zero-Vt, low-Vt, or high-Vt FETs of a type that require a negative V_(GS) to turn OFF but configured so as to not require a negative power supply), series-coupled on at least one end to an “end-cap” FET M₀, where “M₀” designates a FET of a high-Vt type that turns OFF when the V_(GS) of such FET is essentially zero volts. The end-cap M₀ FETs selectably provide either a capacitive DC blocking function or a resistive signal path. Some embodiments may comprise a stack of only M₀ FETs, or a mix of positive-logic FETs and M₀ FETs, so long as at least one end-cap FET is an M₀ FET. Optional end-cap capacitors may be included to prevent early breakdown of corresponding end-cap M₀ FETs.

Some embodiments are configured to withstand high applied RF voltages by including a series-connected bias resistor ladder for the gates of the FETs in the stack (i.e., M1-Mn, M₀), a series-connected Rds drain-source resistor ladder, and either a series-connected or a parallel-connected Rb body charge control resistor ladder. Some embodiments may use series-connected Rb body charge control resistor ladder in conjunction with a parallel-connected bias resistor ladder for the gates. Some embodiments may use a series-connected bias resistor ladder (in a “rail” configuration) in conjunction with a parallel-connected bias resistor ladder (in a “rung” configuration) for the gates.

Some embodiments are configured with one or more AC coupling modules each coupled to at least one end of a corresponding gate bias resistor ladder and/or body charge control resistor ladder, and configured to be coupled to a radio frequency voltage source.

Embodiments of the invention may be used in any circuit configuration in which a FET stack may be necessary or useful, such as RF switch circuits and digitally tuned capacitors (for example, of the type taught in U.S. patent application Ser. No. 15/256,453).

Selectable DC Blocking Embodiments

FIG. 4 is a schematic circuit of an improved positive logic FET stack 400. The stack 400 comprises one or more series-coupled FETs M1-Mn. Each of the FETs M1-Mn are of a type that require a negative V_(GS) to turn OFF (e.g., −3V) but which are configured to not require a negative power supply, as taught in co-pending U.S. patent application Ser. No. 15/256,453. Examples of such FETs M1-Mn may have a V_(T)≈0V and a low R_(ON) (e.g., much less than Z₀ ohms, where Z₀ is the system characteristic impedance).

In some embodiments, the FETs M1-Mn in the “interior” of the stack 400, which may be zero-Vt, low-Vt, or high-Vt FETs, are bracketed by “end-cap” M₀ high-Vt FETs. In other embodiments, only one end of one or more series-coupled positive-logic FETs M1-Mn is series coupled to an M₀ FET; thus, for example, the end-cap M₀ FET in dashed box 402 or the end-cap M₀ FET in dashed box 404 may be omitted in some applications. Zero-Vt, low-Vt and high-Vt FETs are available from a number of different IC commercial foundries. Examples of such end-cap M₀ FETs may have a V_(T)=0.7V, and may have a higher R_(ON) than the FETs M1-Mn.

Each FET M1-Mn, M₀ includes an associated gate resistor Rg; in the illustrated example, the gate resistors Rg are dedicated to each FET and parallel to each other (for other configurations, see below).

Two non-negative supply voltages, Vgate and Vdrain, are used to control the switch state, ON or OFF, of associated FETs M1-Mn, M₀. The Vgate supply voltage is coupled to the gates of the FETs of the stack 400 through corresponding dedicated parallel gate resistors Rg. The Vdrain supply voltage is coupled to a network of parallel resistors Rds that are coupled as shown to respective sources or drains of each of the FETs M1-Mn, including shared drains-source nodes between the FETs M1-Mn. In the embodiment of FIG. 4, the gate resistors Rg and the network resistors Rds may be of equal size, but a person skilled in the art will understand that other embodiments may include resistors of different sizes.

In the embodiment shown in FIG. 4, the drain and source of each end-cap M₀ FET is coupled in parallel with a capacitor Cd. As disclosed in greater detail below, since the end-cap M₀ FETs are biased with Vgs=0V and thus usually cannot handle the full drain-source voltage Vds applied across them, the Vds across each end-cap M₀ FET is lowered (e.g., to 2V) by their respective capacitor Cd.

FIG. 5A is a table showing example voltages for Vgate and Vdrain corresponding to ON and OFF states for the FET stack of FIG. 4. FIG. 5B is a simplified schematic circuit of the FET stack of FIG. 4 showing example drain and source voltages for the interior FETs Mn and end-cap M₀ FETs when in an ON state. FIG. 5C is a simplified schematic circuit of the FET stack of FIG. 4 showing example drain and source voltages for the interior FETs Mn and end-cap M₀ FETs when in an OFF state.

In the example illustrated in FIGS. 4 and 5B, the FETs FET M1-Mn, M₀ are turned ON by applying a positive Vgate voltage (e.g., +3V) to the FET gates through their respective gate resistors Rg, while applying a Vdrain voltage of 0V to the drains and sources of FETs M1-Mn through respective Rds resistors. (Note that, technically, the source for the “top” M₀ FET would be, for example, a coupled antenna or other RF signal source, and thus its drain is at the “bottom” end of that FET. Similarly, the source for the “bottom” M₀ FET would be, for example, circuit ground, and thus its drain is at the “top” end of that FET. Accordingly, the Vdrain voltage is coupled to the respective drains of both the “top” M₀ FET and the “bottom” M₀ FET. As should be clear, “top” and “bottom” are with respect to the illustrated orientation of the FET stack in FIG. 4 and are for convenience only). This configuration effectively creates a positive DC gate-source voltage, V_(GS), for every FET M1-Mn, M₀ (i.e., V_(GS)=+3V with respect to the source voltage of 0V if Vgate=+3V), as shown in FIG. 5B. In the ON state, the end-cap M₀ FETs each present a series resistance of R_(ON). Since the ON-state end-cap M₀ FETs essentially present no series capacitance, shunt tuning inductors L for impedance matching may be omitted, thus improving broadband performance and reducing consumption of IC area.

In the example illustrated in FIGS. 4 and 5C, the FETs FET M1-Mn, M₀ are turned OFF by applying a Vgate voltage of 0V to the FET gates through their respective gate resistors Rg, while applying a positive Vdrain voltage (e.g., +3V) to the drains and sources of FETs M1-Mn (and thus to the source of the “top” M₀ FET, and to the drain of the “bottom” M₀ FET) through respective Rds resistors. This effectively creates a negative V_(GS) for every FET M1-Mn (i.e., V_(GS)=−3V with respect to the source voltage of +3V if Vgate=0V), as shown in FIG. 5C. The end-cap M₀ FETs have either a V_(GS) of −3V (for the “top” M₀ FET) or 0V (for the “bottom” M₀ FET), either of which is sufficient to turn the end-cap M₀ FETs OFF in light of their V_(GS) characteristics (although the current blocking capability of the end-cap M₀ FETs increases as V_(GS) becomes more negative). In the OFF state, the end-cap M₀ FETs each present a series capacitance of C_(OFF), and thus perform a DC blocking function.

As an alternative embodiment, when the “bottom” M₀ FET is ON, the terminal labeled Vdrain in FIG. 4 can be left as an open circuit, since all of the FETs coupled to the Rds resistor network would be coupled to circuit ground through the “bottom” M₀ FET. For the OFF state, the terminal labeled Vdrain in FIG. 4 would be coupled to a suitable voltage (e.g., +3V).

In the example embodiment of FIG. 4, Vdrain is varied between 0V to 3V as a function of the ON or OFF state of the FET stack 400. In other embodiments, Vdrain may be set as a fixed voltage, such as a constant mid-rail voltage (e.g., +1.5V). Thus, similar to the example shown in FIG. 5A-5C, when Vgate is high (e.g., +3V), a positive voltage of +1.5V will appear across the gate-source of each FET M1-Mn, M₀ and turn those FETs ON. Conversely, when Vgate is low (e.g., 0V), a negative voltage of −1.5V will appear across the gate-source of each interior FET M1-Mn and turn those FETs OFF; similarly, V_(GS) for the “top” end-cap M₀ FET will be −1.5V, while the V_(GS) for the “bottom” end-cap M₀ FET will be 0V, and thus both of the end-cap M₀ FETs will be OFF. In a constant Vdrain biasing scheme, lower or higher voltages (e.g., +0.5V) may also be used for Vdrain. In a further embodiment, the fixed mid-rail voltage may be chosen based on the threshold voltage, V_(T), of the FETs to provide a beneficial (potentially optimal) level of gate overdrive (V_(GS)−V_(T)) to balance performance parameters in both ON and OFF states. More generally, the voltages for Vgate and Vdrain can be selected (and potentially optimized) to provide a balanced amount of gate overdrive. If Vdrain is biased to a non-zero DC voltage, a correctly sized shunt capacitor (or RC filter) may need to be added to sufficiently filter out RF voltage swings.

The circuit configuration of the FET stack 400 of FIG. 4 is thus similar to the circuit configurations of the stacks 302, 304 of FIG. 3, except that the end-cap M₀ FETs are used for DC blocking instead of explicit DC blocking capacitors C. In operation, the end-cap M₀ FETs can be turned to an OFF state in which they function as DC blocking capacitors having a capacitance of C_(OFF). Importantly, the end-cap M₀ FETs also can be turned to an ON state that forms an actual resistive DC path rather than a capacitance, resulting in little or no insertion loss, mismatch loss, or frequency dependence penalty compared to constantly present DC blocking capacitors C, and also decreasing switching and settling times. While the R_(ON) for the end-cap M₀ FETs generally may be greater than the R_(ON) for the interior FETs M1-Mn, with larger numbers of interior FETs M1-Mn (i.e., higher stack heights), the R_(ON) of the interior FETs M1-Mn will dominate the series resistance.

A further benefit of using one or more end-cap M₀ FETs as DC blocking capacitances is that they provide automatic ESD protection, because the FET stack is self-protected by drain-to-source breakdown and there are no additional series components between the FET drain-source terminals and the external ports (ESD pulses are applied across external ports). Accordingly, shunt tuning inductors L for ESD protection of MIM capacitors in FET stacks coupled to ports may be omitted, thus reducing consumption of IC area. In addition, using one or more end-cap M₀ FETs provides a reasonable “un-powered OFF” state for applications that benefit from some isolation when the system voltage supply, V_(CC), equals 0V.

In some embodiments, all of the FETs in a stack 400 may be of the M₀ type. In other embodiments, some (but not all) of the interior FETs in a stack 400 may be of the M₀ type, so long as at least one end-cap M₀ FET is provided. For example, the stack 400 may comprise a mix of FETs in a sequence such as M₀-M1-M₀-M2-M₀, where M1 and M2 are positive-logic FETS. Such mixed configurations may have a higher series resistance than the embodiment shown in FIG. 4, due to higher R_(ON) for the M₀ FETs, but would have a lower R_(ON) than a stack comprising all M₀-type FETs. Mixed configurations of FET types and sequences can be used to help optimize the total stack performance parameters of R_(ON), OFF isolation, peak voltage (Vpk) handling, DC operation, and un-powered circuit behavior.

Referring back to FIG. 4, optional capacitors Cd may be coupled in parallel with the end-cap M₀ FETs, from drain to source, as indicated by the dotted connector lines. When the end-cap M₀ FETs are in an ON state, the optional capacitors Cd are effectively bypassed and thus functionally out of the RF signal path. When the end-cap M₀ FETs are in an OFF state, the optional capacitors Cd are coupled to the RF signal path in parallel with the end-cap M₀ FETs (which each present as a capacitance of C_(OFF)). One advantage of the optional capacitors Cd is that they reduce RF voltage swing across the end-cap M₀ FETs (e.g., to 2V instead of a full 3V) and thus help prevent early breakdown of those FETs. It may be desirable for the optional capacitors Cd to have a capacitance that is about 5-10 times the C_(OFF) of the end-cap M₀ FETs. In some embodiments, only the “bottom” M₀ FET (i.e., nearest circuit ground in FIG. 4) includes the capacitor Cd.

With further reference to FIG. 4, the person skilled in the art will appreciate that, regardless of the switching state of the FET stack 400, and except for leakage currents, the FET stack 400 and generation of the bias voltages Vgate and Vdrain does not consume any significant DC power, unlike the case of FET stacks that require a charge pump. This is very beneficial for integrated circuits with stringent power or current consumption requirements. Embodiments may use the existing system voltage supply rail to provide Vgate and Vdrain; however, a person skilled in the art will understand that other design choices to provide Vgate and Vdrain may be envisaged.

In yet other embodiments, the gates of one or both of the end-cap M₀ FETs may be biased separately from the interior FETs M1-Mn. For example, biasing the gates of both end-cap M₀ FETs with a negative gate voltage (and thus a negative V_(GS)) would more completely turn both FETs OFF and improve the linearity and harmonics performance of those FETs, but would not require a significant current draw since such FETs inherently draw low body current and are only two in number. Negative biasing would also allow the optional capacitors Cd to be much smaller than otherwise, or to be omitted entirely. Accordingly, a simple negative voltage generator could be used to generate a negative gate voltage bias for just the end-cap M₀ FETs.

FIG. 6 is a schematic diagram of a series-shunt RF switch circuit 600 using the improved positive logic FET stacks of FIG. 4. In the illustrated example, an RF signal from an antenna ANT is coupled from RF_(IN) through a series stack 602 to RF_(OUT). A shunt stack 604 is coupled between RF_(OUT) and a reference potential (circuit ground in this case). Each of the stacks 602, 604 are similar to the FET stack 400 of FIG. 4, although “n” may differ for the two stacks 602, 604. (To avoid clutter, labels for the gate resistors Rg and the resistors Rds of the network coupled to the sources or drains of each of the FET M1-Mn, M₀ are omitted). As described above with respect to FIG. 4, each of the stacks 602, 604 is configured to be coupled to two non-negative supply voltages, Vgate and Vdrain. Optional capacitors Cd are shown as coupled in parallel with the end-cap M₀ FETs. Notably absent are constantly connected DC blocking capacitors or shunt tuning and ESD protection inductors L, as in FIG. 3.

In a variation of the series-shunt RF switch circuit 600 of FIG. 6, only one end of the series stack 602 and/or the shunt stack 604 includes an end-cap M₀ FET.

Stacked FET Switch Bias Ladders

In the embodiments of the invention disclosed above, Vgate is applied to each FET M1-Mn, M₀ through an associated gate resistor Rg in a conventional biasing scheme, which provides excellent performance in many applications, including low power RF switch circuits and many series and/or shunt circuit configurations. However, for high RF voltage applications, such as RF tuning switches, an innovative biasing scheme provides for an enhanced ability to withstand such voltages.

To better understand the problem posed by application of high RF voltages to a FET stack, FIG. 7 depicts a schematic equivalent circuit of a portion of a prior art FET stack 700, showing parasitic capacitances. The “top” FET, Mtop, of the FET stack 700 is coupled to an RF input voltage V_(RF), and to a next sequential FET, Mnext. As should be appreciated, additional FETs may be similarly sequentially coupled to Mnext.

A conventional parallel gate bias resistor ladder 702 is shown in which a variable DC gate bias voltage Vgate is coupled to the gate of each FET Mtop, Mnext through a corresponding gate resistor Rg. Because the FETs and gate resistors are integrated devices formed on an IC substrate, there are inherent associated parasitic capacitances. In particular, each gate resistor Rg has a parallel parasitic capacitance Cp_(R), and each FET includes parasitic capacitances Cp_(M) between its gate and drain and between its gate and source. Accordingly, the gates of each FET are imperfectly isolated from the gate and source with respect to RF signals.

Using the example of FIG. 7, if V_(RF) is an RF voltage of 100V and the stack 700 is in the OFF state (i.e., all FETs are biased to block drain-source current flow), and if each FET Mtop, Mnext, etc. can safely withstand 3V from drain to source (further assuming that there sufficient other FETs to drop the applied RF voltage such that no one FET is subjected to more than 3V), then the source voltage for Mtop (and thus the drain voltage for Mnext) would be 97V, and the source voltage for Mnext would be 94V; additional FETs in the sequence would similarly drop 3V from drain to source. The parasitic capacitances Cp_(M) will cause imposition of an RF voltage on the gate of Mtop of about 98.5V (i.e., about one-half the difference between 100V−97V); for Mnext, the imposed gate voltage would be about 95.5V. More specifically, the gate resistor Rg for Mtop will essentially have AC ground (0V) on its Vgate side, and very high RF voltage (˜98.5V) on its other side. This causes excessive power dissipation in the gate resistor Rg for Mtop. The parallel parasitic capacitor Cp_(R) across that gate resistor Rg makes the effective resistance even smaller, thereby increasing the RF power dissipation in the gate resistor Rg, which may cause device failure. A similar problem, but at increasingly lower RF voltages, occurs for each of the successive FETs Mnext in the stack 700.

FIG. 8A is a schematic equivalent circuit of a FET stack 800 with a series-connected resistor ladder gate bias scheme in accordance with one aspect of the present invention, showing parasitic capacitances. The “top” FET, Mtop, of the FET stack 800 is coupled to an RF input voltage V_(RF), and to a next sequential FET, Mnext; additional FETs may be similarly sequentially coupled to Mnext. A variable DC gate bias voltage Vgate may be coupled to the gate of each FET Mtop, Mnext at nodes A, B situated after a corresponding one of a plurality of series-coupled gate resistors Rg, which thus form a series-connected gate bias resistor ladder 802. Stated another way, each gate resistor Rg is coupled to the gates of two adjacent FETs, and the series-connected gate bias resistor ladder 802 is configured to be connected to a gate bias voltage Vgate. Accordingly, in the illustrated example, for n FETs, there are n−1 gate resistors Rg (see also FIG. 8B); however, if desired, an additional gate resistor Rg can be coupled between Vgate and the gate of the “bottom” FET in the FET stack 800 (see, for example, resistor R_(BF) in FIG. 8B). Each FET gate will see an applied DC bias voltage of Vgate through the series connection of the gate resistors Rg of the resistor ladder 802; that is, Vgate essentially just charges or discharges each FET gate through corresponding gate resistors Rg.

Using the example values set forth above, if V_(RF) is an RF voltage of 100V and each FET Mtop, Mnext, etc. can safely withstand 3V from drain to source, then the source voltage for Mtop (and thus the drain voltage for Mnext) would be 97V, and the source voltage for Mnext would be 94V. When the stack 800 is in the OFF state, the parasitic capacitances Cp_(M) will enable imposition of an RF voltage on the gate of Mtop of about 98.5V; for Mnext, the imposed gate voltage would be about 95.5V. Importantly, because of the changed configuration of the series-connected gate resistors Rg of the resistor ladder 802, when the stack 800 is in the OFF state, each gate resistor Rg and its corresponding parasitic capacitance Cp_(R) will be subjected to an RF voltage of only 3V, representing the difference between the RF voltages imposed on the gates of adjacent FETs (e.g., 98.5V for the gate of Mtop, and 95.5 for the gate of Mnext). Accordingly, in these examples, while the topmost gate resistor for the FET stack 700 of FIG. 7 is subjected to an RF voltage of 98.5V across its terminals, all gate resistors of the FET stack 800 of FIG. 8 are subjected to an RF voltage of only 3V, and thus are protected from breakdown due to excessive RF voltages capacitively coupled through to the gates of associated FETs.

FIG. 8B is a schematic diagram of a positive logic FET stack 820 with a series-connected gate bias resistor ladder 802 and a series-connected drain-source resistor ladder in accordance with the present invention. The illustrated example is similar to the FET stack 400 of FIG. 4 (with optional capacitors Cd), but the gate resistors Rg are arrayed in a series-connected resistor ladder configuration 802 as in FIG. 8A, and, in addition, the resistors Rds of the drain-source resistor network are also arrayed as a series-connected resistor ladder 804 configured to be connected to a drain-source bias voltage Vdrain. More specifically, each resistor Rds is connected across the drain and source of a corresponding interior FET M1-Mn (but not across the end-cap M₀ FETs, which would defeat the DC blocking state of the end-cap M₀ FETs). As is the case with the series-connected gate bias resistor ladder 802, the series-connected drain-source resistor ladder 804 prevents any of the Rds resistors and FET devices from being subjected to high RF voltages V_(RF). Note that DC blocking capacitors (not shown) may be coupled to Vgate and/or Vdrain as described below with respect to FIG. 8C.

In the example shown in FIG. 8B, the bias voltage Vgate is shown coupled to the “bottom” FET in the FET stack 820 at a “bottom feed” node BF through a “bottom feed” resistor R_(BF) at one end of the series of resistors comprising the gate bias resistor ladder 802. However, alternative connection points may be used if desired; for example, Vgate may be coupled to the series-connected gate bias resistor ladder 802 at a “middle feed” node MF, optionally through a “middle feed” resistor R_(MF). As should be clear, the “middle feed” node MF need not be in the exact middle of the gate bias resistor ladder 802, but may be anywhere between the ends of the series of resistors comprising the gate bias resistor ladder 802. A “middle feed” configuration may be useful when the FET stack 820 could experience an applied RF voltage to FET at the “top” or “bottom” (in relation to FIG. 8B) of the stack 820, as may be the case if the FET stack 820 is designed for a dual-polarity series application (i.e., an RF signal can be applied to either end of the stack) rather than a shunt configuration (see also FIG. 8F below). In particular, a “middle feed” configuration can provide a reduction in power dissipation in the gate resistors Rg regardless of whether an RF voltage amplitude is applied at either end of the transistor stack. Further benefits of a “middle feed” configuration are described in U.S. patent application Ser. No. 15/289,768 referenced above.

In an optional configuration, an additional parallel-connected gate resistor ladder 805 may be included. The parallel-connected gate resistor ladder 805 comprises respective resistors Rg′ (indicated by a block symbol) coupled to the gates of the FETs in the FET stack 820. In some embodiments, a combination of a series-connected gate bias resistor ladder 802 and a parallel-connected gate resistor ladder 805 can provide reduced RF power dissipation while maintaining a low switching time constant (and therefore a high switching speed). Further benefits of a “series plus parallel” gate bias ladder configuration are described in U.S. patent application Ser. No. 15/289,768 referenced above.

FIG. 8C is a schematic diagram of a positive logic FET stack 830 with an added series-connected resistor ladder for controlling body charge, and thus “back gate” characteristics in the FETs. In this example, each FET (of any type M1-Mn, M₀) is treated as an explicit four-terminal transistor, and the body of each of the stacked four terminal FETs is configured to be connected to a body bias voltage Vbody directly or through a corresponding resistor Rb of a body charge control resistor ladder 806. As is the case with the series-connected gate bias resistor ladder 802 of FIG. 8B, the series-connected body charge control resistor ladder 806 prevents any of the resistors Rb from being subjected to high RF voltages V_(RF). A series-connected body charge control resistor ladder 806 is also useful for FET stacks that draw a lot of body current, to mitigate or eliminate interference with gate voltages when high peak RF voltages are applied across the FET stack.

In an optional configuration, an additional parallel-connected gate resistor ladder (not shown, but essentially the same as the parallel gate resistor ladder 805 of FIG. 8C) may be included.

In the circuit configuration of FIG. 8C, a separate Vbody terminal is shown in the schematic diagram for flexibility, but in many applications Vbody can be tied directly to circuit ground. When Vbody is tied directly to circuit ground, all body current in the FETs M1-Mn, M₀ is sourced from circuit ground (e.g., V_(GS)=Vbody=0V). In contrast, in a conventional FET stack of the type shown in FIG. 2, body current needs to be sourced from a negative power supply (e.g., V_(GS)=Vbody=−3V). Such a negative power supply is usually provided by a negative charge pump, the output of which, Vss, will slump when high peak RF voltages are applied across the FET stack and induce significant body current.

In some embodiments, capacitors Cgate, Cdrain, and/or Cbody may be coupled as shown to the Vgate, Vdrain, and Vbody terminals, respectively, and to a reference potential, such as circuit ground. The Cgate, Cdrain, and/or Cbody capacitors, when present, serve to filter out RF voltage from DC bias voltages applied to those terminals (i.e., making Vgate, Vdrain, and/or Vbody AC grounds).

The series-connected gate bias resistor ladder 802 configuration of FIG. 8A, the series-connected drain-source resistor ladder 804 of FIG. 8B, and the series-connected body charge control resistor ladder 806 of FIG. 8C are applicable to conventional FET stacks (e.g., of the types shown in FIG. 2 or 3), as well as to FET switch stacks having one or more end-cap M₀ FETs (i.e., FETs that turn OFF when their V_(GS) is essentially zero volts). For example, FIG. 8D is a schematic diagram of a conventional FET stack 840 with a series-connected gate bias resistor ladder 802, a series-connected drain-source resistor ladder 804, and a series-connected body charge control resistor ladder 806. In contrast to the embodiment of FIG. 8C, the “end-cap” FETs M1 and Mn are conventional FETS, additional drain-source resistors Rds' have been added to the drain-source resistor ladder 804 in order to span the drain and source of each of M1 and Mn, the optional capacitors Cd from FIG. 8C have been removed, and there is no Vdrain bias voltage. This configuration has the advantages for each series-connected resistor ladder 802, 804, 806 noted above for the series-connected gate bias resistor ladder 802, especially protection from breakdown due to excessive capacitively coupled RF voltages.

In some embodiments, the series-connected body charge control resistor ladder 806 may be replaced by a parallel-connected body charge control resistor ladder, either in a conventional FET stack or in a FET stack having one or more end-cap M₀ FETs. For example, FIG. 8E is a schematic diagram of a conventional FET stack 850 with a series-connected gate bias resistor ladder 802, a series-connected drain-source resistor ladder 804, and a parallel-connected body charge control resistor ladder 808. This parallel-connected approach may prove to be advantageous if the body current for a series-connected resistor ladder configuration (as in FIG. 8D) would be high enough that the cumulative IR drop would cause the voltage on the body of the “top” FET Mn (i.e., the FET furthest from the input node for the bias voltage Vbody) to become undesirably high. The parallel-connected body charge control resistor ladder 808 configuration creates a consistent voltage drop across the body resistors Rb such that there is no additive voltage increase.

In an optional configuration, an additional parallel-connected gate resistor ladder (not shown, but essentially the same as the parallel-connected gate resistor ladder 805 of FIG. 8C) may be included. In some embodiments, the configuration of the body charge control resistor ladder may be series-connected (as in FIG. 8D) and the configuration of the gate bias resistor ladder may be parallel-connected (as in FIG. 4) or a combination of series-connected and parallel-connected (as in FIG. 8B, with the added parallel-connected gate resistor ladder 805), with benefits similar to those described above for FIG. 8E.

As should be appreciated, any combination of series-connected resistor ladders 802, 804, 806 may be used in conjunction with a FET stack, with or without adding a parallel-connected gate resistor ladder 805 or substituting a parallel-connected body charge control resistor ladder 808 for a series-connected resistor ladder 806. Examples include: a gate bias resistor ladder plus a body charge control resistor ladder; a gate bias resistor ladder plus a drain-source resistor ladder; a body charge control resistor ladder plus a drain-source resistor ladder; and a gate bias resistor ladder plus a body charge control resistor ladder plus a drain-source resistor ladder.

If more than one resistor ladder is used in a FET stack that includes end-cap M₀ FETs, it may be necessary or useful to provide a non-overlapping sequencing for states changes to the Vgate, Vdrain, and Vbody voltages to help prevent “shoot through” DC current in the end-cap M₀ FETs. Such current may occur because the positive-logic FETs (e.g., FETs M1-Mn in FIG. 4) have different ON-to-OFF and OFF-to-ON switching characteristics and timing compared to the essentially zero V_(GS) M₀ FETs. For example, referring to FIG. 8C, when the FET stack is being switched from the OFF state to the ON state, it may be preferable to change Vdrain (e.g., from 3V to 0V) before changing Vgate (e.g., from 0V to 3V). Conversely, when the FET stack is being switched from the ON state to the OFF state, it may be preferable to change Vgate (e.g., from 3V to 0V) before changing Vdrain (e.g., from 0V to 3V).

Bias Ladders with AC Coupling Modules

The series-connected or series/parallel connected gate resistor ladders described above allow the gate of each FET in a switch stack to track the common mode voltage of its associated FET. In general, such gate resistor ladders must provide a high impedance relative to the impedance (capacitance) between the gate and the source/drain of the FETs in the switch stack. Despite the benefits of a series-connected gate bias resistor ladder 802 (e.g., as shown in FIG. 8A) or of a combination of series-connected gate bias resistor ladder 802 and a parallel-connected gate bias resistor ladder 805 (e.g., as shown in FIG. 8B) as described to this point, in some applications, the FET nearest an applied RF signal (e.g., the “top” FET in FIGS. 8B-8E coupled to V_(RF)) still has an effective RF load that can cause the gate of that FET to not fully track the transistor common mode voltage during RF operation. The RF load on the gate of the “top” FET acts as parasitic capacitance in producing non-ideal voltage division, and if not compensated, can reduce the peak breakdown voltage (BV) withstand capability of that FET by several percent.

However, the load seen by that “top” FET can be substantially mitigated or even totally eliminated by coupling an AC Coupling Gate Module 822 between an RF input terminal and the gate of the FET (through at least a gate resistor) in the stack nearest the RF input terminal. For example, in FIGS. 8B-8E, an AC Coupling Gate Module 822 is coupled between the terminal receiving a V_(RF) signal and the gate of the “top” FET (Mn or M₀, as the case may be)—thus, the AC Coupling Gate Module 822 is also coupled to the “top” of the series-connected gate bias resistor ladder 802.

As another example, FIG. 8F is a schematic diagram of a conventional FET stack 860 with a series-connected gate bias resistor ladder 802 and an AC Coupling Gate Module 822. The FET stack 860 is configured to be coupled in a shunt configuration or in a series configuration. In a shunt configuration, the RF− terminal is usually circuit ground, an RF input signal is applied at the RF+ terminal, and Vgate is applied at the “bottom” of the FET stack 860 through resistor R_(BF). In a series configuration, either or both of the RF+ and RF− terminals may be the source of an RF signal (e.g., RF+ may be coupled between an RF antenna and an amplifier), and Vgate would generally be applied at the “middle” of the FET stack 860 through resistor R_(MF). As noted above, a “middle feed” configuration can provide a reduction in power dissipation in the gate resistors Rg regardless of whether an RF voltage amplitude is applied at either end of the FET stack 860. As should be clear, the FETs M1-Mn of the conventional FET stack 860 of FIG. 8F can be replaced by a positive logic stack of the type shown in FIG. 8B.

In some embodiments, the AC Coupling Gate Module 822 may comprise a DC blocking capacitor C1, while in other embodiments the AC Coupling Gate Module 822 may comprise a DC blocking capacitor C1 connected in series with a resistor R1 (the order of connection of C1 and R1 does not matter, whereas the order of connection does matter for Cgate and R_(BF) at the “bottom” of the stack for a shunt stack configuration). The AC Coupling Gate Module 822 improves voltage division along the gate bias resistor ladder 802 and allows the gate of the “top” FET to more fully track the transistor common mode voltage during RF operation. Another benefit of the AC Coupling Gate Module 822 in a series configuration is that it lets a coupled antenna be loaded directly, rather than through the FETs of the stack.

As noted above, in a series configuration, either or both of the RF+ and RF− terminals may be the source of an RF signal, and Vgate would generally be applied at the “middle” of the FET stack 860 through resistor R_(MF). In such a case, it may be beneficial to couple a Coupling Gate Module between the series-connected gate bias resistor ladder 802 and both of the RF+ and RF− terminals. For example, FIG. 8G is a schematic diagram of a conventional FET stack 870 with a series-connected gate bias resistor ladder 802 and dual AC Coupling Gate Modules. The FET stack 870 of FIG. 8G is similar to the FET stack 860 of FIG. 8F, but with a “middle feed” configuration for Vgate, a first AC Coupling Gate Module 822 at the “top” of the stack nearest FET Mn, and a second AC Coupling Gate Module 822′ at the “bottom” of the stack nearest FET M1. The second AC Coupling Gate Module 822′ may comprise a DC blocking capacitor C2, while in other embodiments the second AC Coupling Gate Module 822′ may comprise a DC blocking capacitor C2 connected in series with a resistor R2. With the illustrated configuration of AC Coupling Gate Modules 822, 822′, the load seen by the FETs nearest the RF+ and RF− terminals can be substantially mitigated or even totally eliminated.

Referring back to FIG. 8F, if a series-connected body charge control resistor ladder 806 is included in the FET stack 860, a similar AC Coupling Body Module 823 will provide the same benefits for the body (or “back gate”) of the “top” FET (in this example, FET Mn, nearest the RF+ terminal). In the illustrated embodiment, the AC Coupling Body Module 823 is coupled between the RF+ terminal and the series-connected body charge control resistor ladder 806 connected to the “top” FET Mn nearest the RF+ terminal. The AC Coupling Body Module 823 may comprise a DC blocking capacitor C2 in some embodiments, while in other embodiments the AC Coupling Body Module 823 may comprise a DC blocking capacitor C2 connected in series with a resistor R2 (again, the order of connection does not matter). Further, in a series configuration, a second AC Coupling Body Module (not shown) may be coupled between the RF− terminal and the series-connected body charge control resistor ladder 806 connected to the “bottom” FET M1 nearest the RF− terminal to provide similar benefits.

An AC Coupling Gate Module and an AC Coupling Body Module may be used independently of each other, or may be used together. In general, when using a series-connected gate bias resistor ladder 802 and/or a series-connected body charge control resistor ladder 806, there are significant benefits to using an AC Coupling Gate and/or Body Module between the respective ladders 802, 806 and any RF terminal presenting an RF load, in order to mitigate or even totally eliminate an RF load across the FET coupled to that RF terminal.

In some embodiments, a parallel-connected gate bias ladder 805 may be included with the values of the resistors Rg′ being selected to isolate for a different voltage division in the gate bias resistor ladders versus the component FETs M1-Mn of the FET stack 860. More particularly, if the voltage division across the FET stack 860 is uniform, then constant Rg values for a series-connected gate bias resistor ladder 802 (i.e., a “rail” only configuration) in conjunction with an AC Coupling Gate Module 822 completely eliminates the RF load on the FETs since the voltage division across the gate bias resistor ladder is uniform. On the other hand, if there is non-ideal voltage division across the FET stack 860, a mismatch occurs between the voltage division across the FETs and the voltage division across the rail-only gate bias resistor ladder. This can be remedied by including a parallel-connected gate bias ladder 805 (i.e., adding “rungs” to the “rails” of the series-connected gate bias resistor ladder 802). Adding rung resistors Rg′ helps reduce the RF load due to non-ideal voltage division across the FET stack 860, with minimal switching time constant penalty.

In some gate bias ladder embodiments having a series-connected gate bias resistor ladder 802 (a “rail” configuration) and a parallel-connected gate bias ladder 805 (a “rung” configuration), there is a benefit to having variable values for the resistors of either or both of the “rail” and “rung” bias resistor ladders 802, 805 (as opposed to constant resistance values). In general, the variation should be tapered from the top of the FET stack to the bottom; that is, the resistor values in one or both of the bias resistor ladders 802, 805 should be higher near the top, tapering to lower at the bottom. For example, compared to constant value rail and rung resistors, there is significant improvement in peak breakdown voltage (BV) for the top-most transistor (nearest the RF signal input) when the rail resistors are variable and the rung resistors are constant, while retaining similar switching time and power dissipation characteristics. In addition, a “variable rail/constant rung” gate bias resistor ladder configuration allows the size of the rung resistors to be much smaller than with other configurations, enabling greater flexibility in designing IC layouts. Similar benefits may be seen in embodiments using a “variable rail/variable rung” gate bias resistor ladder configuration. Note that variable rail resistor values may be adjusted to achieve a desired relationship of (1) the RF load on the FET gates to (2) the peak BV for the top-most transistor. However, there is a tradeoff with the switching time constant.

Note that when the FET stack 860 is coupled to an antenna or the like, then the resistance of R1 is preferably set to one-half of the resistance of the series-connected resistors Rg (i.e., R1=½Rg), since the gate of the “top” FET sees only one-half the voltage below the antenna compared to the voltage across each other FET.

Notably, the inclusion of the AC Coupling Gate Module 822 and/or the AC Coupling Body Module 823 has essentially no adverse effect on the performance characteristics of the FET stack 860 (e.g., in terms of switching time constant, resistor power dissipation, resistive voltage drop through body resistors due to body current, etc.). On the other hand, the inclusion of one or more of such modules 822, 823 substantially eliminates the reduction in the peak breakdown voltage (BV) withstand capability of the “top” FET of the FET stack that would otherwise occur. As one specific example, using modeled circuits to compare (1) a FET stack configuration using a series-connected gate bias resistor ladder 802 but no AC Coupling Gate Module 822 against (2) a FET stack configuration using a series-connected gate bias resistor ladder 802 and an AC Coupling Gate Module 822, the former configuration showed a decrease in BV withstand capability of the “top” FET of about 0.714, while the latter configuration showed a decrease in BV withstand capability of the “top” FET of about 0.001—a factor of over 700 in improvement due to the AC Coupling Gate Module 822.

Note that in a series configuration for the FET stack 860, where either or both of the RF+ and RF− terminals may be the source of an RF signal, an AC Coupling Gate Module 822 may be coupled between a terminal RF+, RF− and a respective end of the series-connected gate bias resistor ladder 802 for the FET stack 860. In such a case, Vgate would normally be applied to the FET stack 860 using a “middle feed” configuration. Similarly, an AC Coupling Body Module 823 may be coupled to each end of the body charge control resistor ladder 806 for the FET stack 860.

Accumulated Charge Sink Structure

Embodiments of the invention, including the embodiments described above, that are fabricated using silicon-on-insulator (SOI)—including silicon-on-sapphire (SOS) MOSFETs—may benefit by adding an accumulated charge sink (ACS) structure to one or more constituent FETs (i.e., M1-Mn and/or M₀). In particular, the linearity of an SOI MOSFET can be improved by providing a means for sweeping out accumulated charge trapped below the gate oxide of the MOSFET, such as by use of the “HaRP”™ accumulated charge sink (ACS) technology taught in U.S. Pat. No. 7,910,993, issued Mar. 22, 2011, entitled “Method and Apparatus for use in Improving Linearity of MOSFETs Using an Accumulated Charge Sink”, which is hereby incorporated by reference. An “accumulated charge”, as defined in more detail in U.S. Pat. No. 7,910,993 and as used herein, refers to gate-bias induced carriers that may accumulate in the body of an OFF-state MOSFET, even if the majority carriers in the body do not have the same polarity as the accumulated charge.

More particularly, as described in greater detail in U.S. Pat. No. 7,910,993, no matter what mode of operation an SOI MOSFET employs (i.e., whether enhancement or depletion mode), when the MOSFET is operated in an OFF-state (i.e., the gate voltage does not exceed V_(th)), and when a sufficient nonzero gate bias voltage is applied with respect to the MOSFET source and drain, an “accumulated charge” may occur under the MOSFET gate which adversely affects the linearity and other aspects of the performance of the MOSFET. To alleviate the effects of such accumulated charge, an SOI MOSFET may include an ACS structure that allows discharge of accumulated charge when the MOSFET is in a non-conducting (OFF) mode while allowing normal operation when the MOSFET is in a conducting (ON) mode.

FIG. 8H is a simplified schematic diagram of an SOI MOSFET 810 that includes an ACS structure along with conventional drain D, source S, and gate G terminals and a gate resistor Rg. In this particular illustrated example, the ACS structure comprises a diode 812 coupled from the body of the MOSFET 810 to the gate G of the MOSFET 810. The diode 812 discharges accumulated charge from the body of the MOSFET 810 through the gate G when the MOSFET 810 is in a non-conducting (OFF) mode, while allowing normal operation when the MOSFET 810 is in a conducting (ON) mode. Alternative ACS structure circuits and methods for reducing accumulated charge in floating body semiconductor devices are described in greater detail in U.S. Pat. No. 7,910,993 referenced above. The MOSFET 810 along with an ACS structure (e.g., a body-to-gate diode 812) may also be referred to as an “ACS FET”. As should be clear, in an SOI MOSFET stack comprising one or more series-coupled positive-logic MOSFETs M1-Mn (i.e., FETs not requiring a negative voltage supply to turn OFF), coupled to one or more series-coupled “end-cap” M₀ MOSFETs, one or more of the MOSFETs may be ACS FETs.

Methods

Note that in specifying a particular circuit embodiment having the architecture and characteristics of the example circuits described above, the principal of “superposition” may be used as a method for optimizing DC biasing and best AC voltage handling configuration independently, and then combining the results. That is, one can first define an optimum circuit configuration independently for AC behavior and DC behavior, and then overlay and merge the two solutions. Mathematically, superposition may be expressed as f(DC+AC)=f(DC)+f(AC), where “DC” is the DC behavior of the circuit, and “AC” is the AC behavior of the circuit.

Another aspect of the invention includes methods for providing a FET switch stack. For example, FIG. 9 is a process flow diagram 900 showing one method for providing a FET switch stack. The method includes: providing one or more series-coupled positive-logic FETs (STEP 902); and providing a first end-cap FET of a type that turns OFF when the V_(GS) of such end-cap FET is essentially zero volts, series-coupled to a first end of the one or more series-coupled positive-logic FETs (STEP 904).

Additional aspects of the method of FIG. 9 may include one or more of the following: providing a second end-cap FET of a type that turns OFF when the V_(GS) of such end-cap FET is essentially zero volts, series-coupled to a second end of the one or more series-coupled positive-logic FETs; further including series coupling, to the FET switch stack, at least one additional FET of the type that turns OFF when the VGS of such FET is essentially zero volts; wherein each end-cap FET functions as a DC blocking capacitor when in an OFF state, and as a resistive signal path when in an ON state; further including coupling at least one capacitor between the source and the drain of a corresponding end-cap FET; further including providing a gate bias resistor ladder including a plurality of series-coupled resistors configured to be coupled to a gate bias voltage, wherein each resistor is coupled to the respective gates of corresponding adjacent FETs; further including a capacitor coupled between the gate bias resistor ladder and a reference potential; further including coupling an AC coupling gate module to at least one end of the gate bias resistor ladder and configuring the AC coupling gate module to be coupled to a radio frequency voltage source; wherein the AC coupling gate module includes one of a capacitor or a capacitor series coupled to a resistor; further including providing a body charge control resistor ladder including a plurality of series-coupled or parallel-coupled resistors configured to be coupled to a body bias voltage, wherein each resistor is coupled to the body of at least one corresponding FET; further including a capacitor coupled between the body charge control resistor ladder and a reference potential; further including coupling an AC coupling body module to at least one end of the body charge control resistor ladder, the AC coupling body module configured to be coupled to a radio frequency voltage source; wherein the AC coupling body module includes one of a capacitor or a capacitor series coupled to a resistor; further including providing a drain-source resistor ladder including a plurality of series-coupled resistors configured to be coupled to a drain-source bias voltage, wherein each resistor is coupled to the respective drains and sources of corresponding adjacent positive-logic FETs; further including coupling a negative gate bias voltage supply to the gates of the end-cap FETs; and wherein at least one FET is an ACS FET.

As another method example, FIG. 10 is a process flow diagram 1000 showing one method for providing a series-shunt switch circuit. The method includes: providing a series FET switch stack having an input port configured to be coupled in series with an RF signal line, and an output port (STEP 1002); coupling a shunt FET switch stack to the output port of the series FET switch stack (STEP 1004); configuring the shunt FET switch stack to be coupled to a reference potential (STEP 1006); wherein the series FET switch stack and the shunt FET switch stack have at least one end-cap FET that turns OFF when the V_(GS) of such end-cap FET is essentially zero volts (STEP 1008), or wherein the series FET switch stack and the shunt FET switch stack include at least one series-coupled positive-logic FET, series-coupled to at least one end-cap FET of a type that turns OFF when the V_(GS) of such end-cap FET is essentially zero volts (STEP 1010).

As yet another method example, FIG. 11 is a process flow diagram 1100 showing one method for biasing a FET switch stack. The method includes: providing a plurality of series-coupled FETs (STEP 1102); providing a gate bias resistor ladder including a plurality of resistors configured to be coupled to a gate bias voltage and to the gates of at least one corresponding FET (STEP 1104); providing a body charge control resistor ladder including a plurality of resistors configured to be coupled to a body bias voltage and to the body of at least one corresponding FET (STEP 1106); and providing a drain-source resistor ladder including a plurality of series-coupled resistors configured to be coupled to a drain-source bias voltage, wherein each resistor is coupled to the respective drains and sources of at least one corresponding adjacent FET (STEP 1108); and wherein the resistors of the gate bias resistor ladder are series-connected and the resistors of the body charge control resistor ladder are parallel-connected (STEP 1110), or alternatively, wherein the resistors of the gate bias resistor ladder are parallel-connected and the resistors of the body charge control resistor ladder are series-connected (STEP 1112).

Additional aspects of the method of FIG. 11 may include one or more of the following: wherein the plurality of series-coupled FETs includes one or more series-coupled positive-logic FET, series-coupled on at least one end to an end-cap FET of a type that turns OFF when the V_(GS) of such end-cap FET is essentially zero volts; further including a second end-cap FET of a type that turns OFF when the VGS of such end-cap FET is essentially zero volts, series-coupled to a second end of the one or more series-coupled positive-logic FETs; coupling a capacitor between the gate bias resistor ladder and a reference potential; coupling an AC coupling gate module to at least one end of the gate bias resistor ladder and configuring the AC coupling gate module to be coupled to a radio frequency voltage source; wherein the AC coupling gate module includes one of a capacitor or a capacitor series coupled to a resistor; further including coupling an AC coupling body module coupled to at least one end of the body charge control resistor ladder and configuring the AC coupling gate module to be coupled to a radio frequency voltage source; wherein the AC coupling body module includes one of a capacitor or a capacitor series coupled to a resistor; coupling a capacitor between the body charge control resistor ladder and a reference potential; configuring the gate bias resistor ladder to be coupled to a bias voltage at a node at a first end of the gate bias resistor ladder; configuring the gate bias resistor ladder to be coupled to a bias voltage at a node between a first end and a second end of the gate bias resistor ladder; wherein at least one FET is an ACS FET; further including coupling a parallel-connected gate resistor ladder, including a plurality of resistors, between the gate bias resistor ladder and the gates of corresponding FETs; wherein the plurality of series-coupled FETs includes at least one series-coupled positive-logic FET, bracketed by series-coupled end-cap FETs of a type that turn OFF when the V_(GS) of such end-cap FETs is essentially zero volts; and wherein the plurality of series-coupled FETs includes a first end-cap FET of a type that turns OFF when the V_(GS) of such end-cap FET is essentially zero volt, a second end-cap FET of a type that turns OFF when the V_(GS) of such end-cap FET is essentially zero volts, and one or more series-coupled positive-logic FETs, series coupled between the first end-cap FET and the second end-cap FET.

As still another method example, FIG. 12 is a process flow diagram 1200 showing a method for providing a FET switch stack. The method includes: providing a plurality of series-coupled FETs (STEP 1202); coupling a gate bias resistor ladder to the gates of the series-coupled FETs (STEP 1204); coupling an AC coupling gate module to at least one end of the gate bias resistor ladder and configuring the AC coupling gate module to be coupled to a corresponding radio frequency voltage source (STEP 1206); optionally, coupling a body charge control resistor ladder to the bodies of the series-coupled FETs (STEP 1208); and, optionally, coupling an AC coupling body module to at least one end of the body charge control resistor ladder and configured to be coupled to the corresponding radio frequency voltage source (STEP 1210).

Additional aspects of the method of FIG. 12 may include one or more of the following: wherein the plurality of series-coupled FETs includes one or more series-coupled positive-logic FET, series-coupled on at least one end to an end-cap FET of a type that turns OFF when the V_(GS) of such end-cap FET is essentially zero volts; further including providing a second end-cap FET of a type that turns OFF when the VGS of such end-cap FET is essentially zero volts, series-coupled to a second end of the one or more series-coupled positive-logic FETs; further including providing a drain-source resistor ladder including a plurality of series-coupled resistors configured to be coupled to a drain-source bias voltage, wherein each resistor is coupled to the respective drains and sources of corresponding adjacent positive-logic FETs; wherein the gate bias resistor ladder includes a plurality of series-connected resistors; wherein the body charge control resistor ladder includes a plurality of series-connected resistors; wherein the AC coupling gate module includes one of a capacitor or a capacitor series coupled to a resistor; wherein the AC coupling body module includes one of a capacitor or a capacitor series coupled to a resistor; wherein the gate bias resistor ladder is configured to be coupled to a bias voltage at a node at a first end of the gate bias resistor ladder; wherein the gate bias resistor ladder is configured to be coupled to a bias voltage at a node between a first end and a second end of the gate bias resistor ladder; and wherein at least one FET is an ACS FET.

As yet another method example, FIG. 13 is a process flow diagram 1300 showing another method for biasing a FET switch stack. The method includes: providing a plurality of series-coupled FETs (STEP 1302); providing a first gate resistor ladder including a plurality of parallel-connected constant-valued (or, optionally, variable-valued) resistors each coupled to the gate of one corresponding FET (STEP 1304); and providing a second gate bias resistor ladder including a plurality of series-connected variable-valued resistors each coupled to one corresponding constant-valued resistor of the first gate resistor ladder (STEP 1306).

Fabrication Technologies & Options

The term “MOSFET”, as used in this disclosure, means any field effect transistor (FET) with an insulated gate and comprising a metal or metal-like, insulator, and semiconductor structure. The terms “metal” or “metal-like” include at least one electrically conductive material (such as aluminum, copper, or other metal, or highly doped polysilicon, graphene, or other electrical conductor), “insulator” includes at least one insulating material (such as silicon oxide or other dielectric material), and “semiconductor” includes at least one semiconductor material.

As should be readily apparent to one of ordinary skill in the art, various embodiments of the invention can be implemented to meet a wide variety of specifications. Unless otherwise noted above, selection of suitable component values is a matter of design choice and various embodiments of the invention may be implemented in any suitable IC technology (including but not limited to MOSFET structures), or in hybrid or discrete circuit forms. Integrated circuit embodiments may be fabricated using any suitable substrates and processes, including but not limited to standard bulk silicon, silicon-on-insulator (SOI), and silicon-on-sapphire (SOS). Unless otherwise noted above, the invention may be implemented in other transistor technologies such as bipolar, GaAs HBT, GaN HEMT, GaAs pHEMT, and MESFET technologies. However, the inventive concepts described above are particularly useful with an SOI-based fabrication process (including SOS), and with fabrication processes having similar characteristics. Fabrication in CMOS on SOI or SOS processes enables circuits with low power consumption, the ability to withstand high power signals during operation due to FET stacking, good linearity, and high frequency operation (i.e., radio frequencies up to and exceeding 50 GHz). Monolithic IC implementation is particularly useful since parasitic capacitances generally can be kept low (or at a minimum, kept uniform across all units, permitting them to be compensated) by careful design.

Voltage levels may be adjusted, or voltage and/or logic signal polarities reversed, depending on a particular specification and/or implementing technology (e.g., NMOS, PMOS, or CMOS, and enhancement mode or depletion mode transistor devices). Component voltage, current, and power handling capabilities may be adapted as needed, for example, by adjusting device sizes, serially “stacking” components to withstand greater voltages as described above, and/or using multiple components in parallel to handle greater currents. Additional circuit components may be added to enhance the capabilities of the disclosed circuits and/or to provide additional functional without significantly altering the functionality of the disclosed circuits.

CONCLUSION

A number of embodiments of the invention have been described. It is to be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, some of the steps described above may be order independent, and thus can be performed in an order different from that described. Further, some of the steps described above may be optional. Various activities described with respect to the methods identified above can be executed in repetitive, serial, or parallel fashion.

It is to be understood that the foregoing description is intended to illustrate and not to limit the scope of the invention, which is defined by the scope of the following claims, and that other embodiments are within the scope of the claims. (Note that the parenthetical labels for claim elements are for ease of referring to such elements, and do not in themselves indicate a particular required ordering or enumeration of elements; further, such labels may be reused in dependent claims as references to additional elements without being regarded as starting a conflicting labeling sequence). 

What is claimed is:
 1. A FET switch stack, including: (a) a plurality of series-coupled FETs defining a signal path and including one or more series-coupled positive-logic FETs requiring a relative negative V_(GS) to effectively turn OFF but configured to not require a negative voltage source; (b) a first end-cap FET that turns OFF when the V_(GS) of such first end-cap FET is essentially zero volts, series-coupled to a first end of the plurality of series-coupled FETs in the signal path and configured to selectably provide either a capacitive DC blocking function or a resistive signal path; (c) a gate bias resistor ladder including a plurality of resistors configured to be coupled to a gate bias voltage and to the gate of at least one corresponding series-coupled FET or the first end-cap FET; and (d) a body charge control resistor ladder including a plurality of resistors configured to be coupled to a body bias voltage and to the body of at least one corresponding series-coupled FET; wherein the resistors of the gate bias resistor ladder are series-connected and the resistors of the body charge control resistor ladder are parallel-connected.
 2. The invention of claim 1, further including a second end-cap FET that turns OFF when the V_(GS) of such second end-cap FET is essentially zero volts, series-coupled to a second end of the plurality of series-coupled FETs in the signal path and configured to selectably provide either a capacitive DC blocking function or a resistive signal path.
 3. The invention of claim 1, further including an AC coupling gate module coupled to at least one end of the gate bias resistor ladder and configured to be coupled to a radio frequency voltage source.
 4. The invention of claim 3, wherein the AC coupling gate module includes one of a capacitor or a capacitor series coupled to a resistor.
 5. The invention of claim 1, further including an AC coupling body module coupled to at least one end of the body charge control resistor ladder and configured to be coupled to a radio frequency voltage source.
 6. The invention of claim 5, wherein the AC coupling body module includes one of a capacitor or a capacitor series coupled to a resistor.
 7. The invention of claim 1, further including a capacitor coupled between the gate bias resistor ladder and a reference potential.
 8. The invention of claim 1, further including a capacitor coupled between the body charge control resistor ladder and a reference potential.
 9. The invention of claim 1, further including a parallel-connected gate resistor ladder, including a plurality of resistors each coupled between the gate bias resistor ladder and the gate of a corresponding FET.
 10. The invention of claim 1, further including a drain-source resistor ladder including a plurality of series-coupled resistors configured to be coupled to a drain-source bias voltage, wherein each resistor is coupled to the respective drains and sources of at least one corresponding adjacent series-coupled FET.
 11. A FET switch stack, including: (a) a plurality of series-coupled FETs defining a signal path and including one or more series-coupled positive-logic FETs requiring a relative negative V_(GS) to effectively turn OFF but configured to not require a negative voltage source; (b) a first end-cap FET that turns OFF when the V_(GS) of such first end-cap FET is essentially zero volts, series-coupled to a first end of the plurality of series-coupled FETs in the signal path and configured to selectably provide either a capacitive DC blocking function or a resistive signal path; (c) a gate bias resistor ladder including a plurality of resistors configured to be coupled to a gate bias voltage and to the gate of at least one corresponding series-coupled FET or the first end-cap FET; and (d) a body charge control resistor ladder including a plurality of resistors configured to be coupled to a body bias voltage and to the body of at least one corresponding series-coupled FET; wherein the resistors of the gate bias resistor ladder are parallel-connected and the resistors of the body charge control resistor ladder are series-connected.
 12. The invention of claim 11, further including a second end-cap FET that turns OFF when the V_(GS) of such second end-cap FET is essentially zero volts, series-coupled to a second end of the plurality of series-coupled FETs in the signal path and configured to selectably provide either a capacitive DC blocking function or a resistive signal path.
 13. The invention of claim 11, further including an AC coupling gate module coupled to at least one end of the gate bias resistor ladder and configured to be coupled to a radio frequency voltage source.
 14. The invention of claim 13, wherein the AC coupling gate module includes one of a capacitor or a capacitor series coupled to a resistor.
 15. The invention of claim 11, further including an AC coupling body module coupled to at least one end of the body charge control resistor ladder and configured to be coupled to a radio frequency voltage source.
 16. The invention of claim 15, wherein the AC coupling body module includes one of a capacitor or a capacitor series coupled to a resistor.
 17. The invention of claim 11, further including a capacitor coupled between the gate bias resistor ladder and a reference potential.
 18. The invention of claim 11, further including a capacitor coupled between the body charge control resistor ladder and a reference potential.
 19. The invention of claim 11, further including a series-connected gate resistor ladder, including a plurality of resistors configured to be coupled to a gate bias voltage, each resistor configured to be coupled to a corresponding resistor of the gate bias resistor ladder.
 20. The invention of claim 11, further including a drain-source resistor ladder including a plurality of series-coupled resistors configured to be coupled to a drain-source bias voltage, wherein each resistor is coupled to the respective drains and sources of at least one corresponding adjacent series-coupled FET. 